The following functions deal with making a connection to a
PostgreSQL backend server. An
application program can have several backend connections open at
one time. (One reason to do that is to access more than one
database.) Each connection is represented by a PGconn
object, which is obtained from the function PQconnectdb or PQsetdbLogin. Note that these functions will
always return a non-null object pointer, unless perhaps there is
too little memory even to allocate the PGconn object. The PQstatus function should be called to check
whether a connection was successfully made before queries are
sent via the connection object.

PQconnectdb

Makes a new connection to the database server.

PGconn *PQconnectdb(const char *conninfo);

This function opens a new database connection using the
parameters taken from the string conninfo. Unlike PQsetdbLogin below, the parameter set can
be extended without changing the function signature, so use
of this function (or its nonblocking analogues PQconnectStart and PQconnectPoll) is preferred for new
application programming.

The passed string can be empty to use all default
parameters, or it can contain one or more parameter
settings separated by whitespace. Each parameter setting is
in the form keyword = value.
Spaces around the equal sign are optional. To write an
empty value or a value containing spaces, surround it with
single quotes, e.g., keyword = 'a
value'. Single quotes and backslashes within the value
must be escaped with a backslash, i.e., \' and \\.

The currently recognized parameter key words are:

host

Name of host to connect to. If this begins with a slash, it
specifies Unix-domain communication rather than
TCP/IP communication; the value is the name of the
directory in which the socket file is stored. The
default behavior when host
is not specified is to connect to a Unix-domain
socket in
/tmp (or whatever socket
directory was specified when PostgreSQL was built). On
machines without Unix-domain sockets, the default is
to connect to localhost.

hostaddr

Numeric IP address of host to connect to. This
should be in the standard IPv4 address format, e.g.,
172.28.40.9. If your machine
supports IPv6, you can also use those addresses.
TCP/IP communication is always used when a nonempty
string is specified for this parameter.

Using hostaddr instead of
host allows the application
to avoid a host name look-up, which may be important
in applications with time constraints. However,
Kerberos authentication requires the host name. The
following therefore applies: If host is specified without hostaddr, a host name lookup occurs.
If hostaddr is specified
without host, the value for
hostaddr gives the remote
address. When Kerberos is used, a reverse name query
occurs to obtain the host name for Kerberos. If both
host and hostaddr are specified, the value for
hostaddr gives the remote
address; the value for host
is ignored, unless Kerberos is used, in which case
that value is used for Kerberos authentication. (Note
that authentication is likely to fail if libpq is passed a host name that
is not the name of the machine at hostaddr.) Also, host rather than hostaddr is used to identify the
connection in ~/.pgpass
(see Section
29.13).

Without either a host name or host address,
libpq will connect
using a local Unix-domain socket; or on machines
without Unix-domain sockets, it will attempt to
connect to localhost.

port

Port number to connect to at the server host, or
socket file name extension for Unix-domain
connections.

dbname

The database name. Defaults to be the same as the
user name.

user

PostgreSQL user
name to connect as. Defaults to be the same as the
operating system name of the user running the
application.

password

Password to be used if the server demands password
authentication.

connect_timeout

Maximum wait for connection, in seconds (write as
a decimal integer string). Zero or not specified
means wait indefinitely. It is not recommended to use
a timeout of less than 2 seconds.

options

Command-line options to be sent to the server.

tty

Ignored (formerly, this specified where to send
server debug output).

sslmode

This option determines whether or with what
priority an SSL
connection will be negotiated with the server. There
are four modes: disable will
attempt only an unencrypted SSL connection; allow will negotiate, trying first a
non-SSL
connection, then if that fails, trying an
SSL connection;
prefer (the default) will
negotiate, trying first an SSL connection, then if that
fails, trying a regular non-SSL connection; require will try only an
SSL
connection.

If PostgreSQL is
compiled without SSL support, using option require will cause an error, while
options allow and prefer will be accepted but
libpq will not in
fact attempt an SSL connection.

requiressl

This option is deprecated in favor of the
sslmode setting.

If set to 1, an SSL connection to the server is
required (this is equivalent to sslmoderequire). libpq will then refuse to
connect if the server does not accept an
SSL connection. If
set to 0 (default), libpq will negotiate the
connection type with the server (equivalent to
sslmodeprefer). This option is only available
if PostgreSQL is
compiled with SSL support.

krbsrvname

Kerberos service name to use when authenticating
with Kerberos 5. This must match the service name
specified in the server configuration for Kerberos
authentication to succeed. (See also Section
20.2.3.)

service

Service name to use for additional parameters. It
specifies a service name in pg_service.conf that holds additional
connection parameters. This allows applications to
specify only a service name so connection parameters
can be centrally maintained. See Section 29.14.

If any parameter is unspecified, then the corresponding
environment variable (see Section 29.12) is checked. If the
environment variable is not set either, then the indicated
built-in defaults are used.

This is the predecessor of PQconnectdb with a fixed set of
parameters. It has the same functionality except that the
missing parameters will always take on default values.
Write NULL or an empty string for
any one of the fixed parameters that is to be
defaulted.

This is a macro that calls PQsetdbLogin with null pointers for the
login and pwd parameters. It is provided for
backward compatibility with very old programs.

PQconnectStartPQconnectPoll

Make a connection
to the database server in a nonblocking manner.

PGconn *PQconnectStart(const char *conninfo);

PostgresPollingStatusType PQconnectPoll(PGconn *conn);

These two functions are used to open a connection to a
database server such that your application's thread of
execution is not blocked on remote I/O whilst doing so. The
point of this approach is that the waits for I/O to
complete can occur in the application's main loop, rather
than down inside PQconnectdb,
and so the application can manage this operation in
parallel with other activities.

The database connection is made using the parameters
taken from the string conninfo,
passed to PQconnectStart.
This string is in the same format as described above for
PQconnectdb.

Neither PQconnectStart nor
PQconnectPoll will block, so
long as a number of restrictions are met:

The hostaddr and host parameters are used appropriately
to ensure that name and reverse name queries are not
made. See the documentation of these parameters under
PQconnectdb above for
details.

If you call PQtrace,
ensure that the stream object into which you trace will
not block.

You ensure that the socket is in the appropriate
state before calling PQconnectPoll, as described
below.

To begin a nonblocking connection request, call
conn = PQconnectStart("connection_info_string"). If
conn is null, then libpq has been unable to allocate a
new PGconn structure.
Otherwise, a valid PGconn
pointer is returned (though not yet representing a valid
connection to the database). On return from PQconnectStart, call status = PQstatus(conn). If status equals CONNECTION_BAD, PQconnectStart has failed.

If PQconnectStart
succeeds, the next stage is to poll libpq so that it may proceed with the
connection sequence. Use PQsocket(conn) to obtain the descriptor
of the socket underlying the database connection. Loop
thus: If PQconnectPoll(conn)
last returned PGRES_POLLING_READING, wait until the socket
is ready to read (as indicated by select(), poll(), or similar system function). Then
call PQconnectPoll(conn)
again. Conversely, if PQconnectPoll(conn) last returned
PGRES_POLLING_WRITING, wait until
the socket is ready to write, then call PQconnectPoll(conn) again. If you have
yet to call PQconnectPoll,
i.e., just after the call to PQconnectStart, behave as if it last
returned PGRES_POLLING_WRITING.
Continue this loop until PQconnectPoll(conn) returns PGRES_POLLING_FAILED, indicating the
connection procedure has failed, or PGRES_POLLING_OK, indicating the connection
has been successfully made.

At any time during connection, the status of the
connection may be checked by calling PQstatus. If this gives CONNECTION_BAD, then the connection procedure
has failed; if it gives CONNECTION_OK, then the connection is
ready. Both of these states are equally detectable from the
return value of PQconnectPoll, described above. Other
states may also occur during (and only during) an
asynchronous connection procedure. These indicate the
current stage of the connection procedure and may be useful
to provide feedback to the user for example. These statuses
are:

CONNECTION_STARTED

Waiting for connection to be made.

CONNECTION_MADE

Connection OK; waiting to send.

CONNECTION_AWAITING_RESPONSE

Waiting for a response from the server.

CONNECTION_AUTH_OK

Received authentication; waiting for backend
start-up to finish.

CONNECTION_SSL_STARTUP

Negotiating SSL encryption.

CONNECTION_SETENV

Negotiating environment-driven parameter
settings.

Note that, although these constants will remain (in
order to maintain compatibility), an application should
never rely upon these occurring in a particular order, or
at all, or on the status always being one of these
documented values. An application might do something like
this:

The connect_timeout connection
parameter is ignored when using PQconnectPoll; it is the application's
responsibility to decide whether an excessive amount of
time has elapsed. Otherwise, PQconnectStart followed by a PQconnectPoll loop is equivalent to
PQconnectdb.

Note that if PQconnectStart returns a non-null
pointer, you must call PQfinish when you are finished with it,
in order to dispose of the structure and any associated
memory blocks. This must be done even if the connection
attempt fails or is abandoned.

Returns a connection options array. This may be used to
determine all possible PQconnectdb options and their current
default values. The return value points to an array of
PQconninfoOption structures,
which ends with an entry having a null keyword pointer. The null pointer is
returned if memory could not be allocated. Note that the
current default values (val
fields) will depend on environment variables and other
context. Callers must treat the connection options data as
read-only.

After processing the options array, free it by passing
it to PQconninfoFree. If this
is not done, a small amount of memory is leaked for each
call to PQconndefaults.

PQfinish

Closes the connection to the server. Also frees memory
used by the PGconn object.

void PQfinish(PGconn *conn);

Note that even if the server connection attempt fails
(as indicated by PQstatus),
the application should call PQfinish to free the memory used by the
PGconn object. The PGconn pointer must not be used again
after PQfinish has been
called.

PQreset

Resets the communication channel to the server.

void PQreset(PGconn *conn);

This function will close the connection to the server
and attempt to reestablish a new connection to the same
server, using all the same parameters previously used. This
may be useful for error recovery if a working connection is
lost.

PQresetStartPQresetPoll

Reset the communication channel to the server, in a
nonblocking manner.

int PQresetStart(PGconn *conn);

PostgresPollingStatusType PQresetPoll(PGconn *conn);

These functions will close the connection to the server
and attempt to reestablish a new connection to the same
server, using all the same parameters previously used. This
may be useful for error recovery if a working connection is
lost. They differ from PQreset (above) in that they act in a
nonblocking manner. These functions suffer from the same
restrictions as PQconnectStart and PQconnectPoll.

To initiate a connection reset, call PQresetStart. If it returns 0, the reset
has failed. If it returns 1, poll the reset using
PQresetPoll in exactly the
same way as you would create the connection using
PQconnectPoll.